Just curious if I can use a spare BB solid roller to measure pushrod length for a sb with a hydraulic roller? I have alway read to use a solid to measure pushrod length, or can I do it with the hydraulic roller lifters for my 408 sb build up?

Nope.
Hyd rollers use a shorter pushrod than solids or solid rollers.
Put a light spring on & check the pattern with an adjustable pushrod or adjust the pushrod so the closed pattern is just to the intake side of center on the valve stem & you will be very close ;)

There's dozens of different height of solid roller lifters. They're not "standardized" like flat or hyd roller lifters are.

Your best bet is to pack one of the lifters you're going to use with Vaseline, and use light checking valve springs like wolfplace said.

Best adjustment is where the lifter tip moves the least distance across the valve stem as it goes through its cycle. That means there's the least side loading on the valve guide and the rocker is about centered in its range. Doesn't matter so much where on the stem the pattern fallfs, just that it's as small as possible. Use gear marking compound or white lithium grease to creat the pattern on the valve stem.